On-load transformer tap changer



June 23, 1964 w. LEPPER 3,138,671

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June 23, 1964 w. LEPPER ON-LOAD TRANSFORMER TAP CHANGER 8 Sheets-Sheet 8Filed Dec. 9, 1959 United States Patent 3,138,671 ON-LOAD TRANSFORMERTAP CHAN GER Wilhelm Lepper, Berghaus Heckenfels, Honnef (Rhine),Germany Filed Dec. 9, 1959, Ser. No. 858,461 Claims priority,application Germany Jan. 7, 1959 4 Claims. (Cl. 200-11) This inventionrelates to an on-load tap changer for transformers, comprising a bank ofcontacts (tap contacts) on a stationary contact rail, each connectedwith a transformer tapping,'and at least one switching assembly with twoon-load tap selector and load-transferring contacts which move from tapto tap by first preparing the change by introducing a divertor member,such as a resistor, into the tapped circuit which is about to beselected, by then transferring the load by rupturing the existing tappedcircuit and by finally completing the change by bridging or shortcircuiting the divertor resistor in the freshly selected tapped circuit.

In on-load tap changers hitherto known to the art the two functionsinvolved in effecting a change-over from tap to tap, namely that ofselecting the next tap and that of actually transferring the load, areperformed by two separate switching elements, the selector element whichprepares the change independently of the existing operational positionand the generally separate load-transferring switching elementoften evenhoused in a casing of its ownwhich transfers the load to the new tapindependently of the selector element.

The major disadvantages inherent in these known forms of constructionprincipally consist in their structural complexity and their consequenthigh production cost. Moreover, the accommodation of the selector andload-transferring switch elements in separate housings undesirablyincreases the proportions and spatial requirements of such apparatus.Furthermore, in known forms of construction maintenance and the possiblynecessary replacement of components of the on-load tap changer areextremely complicated.

The present invention therefore contemplates a solution of the problemof overcoming the defects of known designs and of developing a type ofon-load tap changing equipment which is simple and cheap to produce,which is more compact and requires much less space, and which is capableof high-speed operation since there is less wear of the contacts byburning. Moreover, the contemplated on-load tap changer is desired to befault-free in operation-even in the event of failures in the network-andit should be capable of electric remote control.

The contemplated objects are achieved by the invention in that themovable switching assembly of the on-load tap changer comprises twoswitching elements which are independently movable in relation to thecontacts of the stationary contact rail, and which in consecutive tapchanges alternately perform the functions of preparing the change and oftransferring the load.

This dual function of the switching elements of an on-load tap changeraccording to the invention permits the entire equipment to be ofextremely simple and compact design. Apart from the fact that separatehousings for a selector and a load-transferring switch means are notrequired, the necessary arrangements for driving the switching elementscan be simplified considerably. The outstanding advantage afforded bythe solution proposed by the present invention, in which the switchingelements alternately perform the twin functions of preparing thechange-over and of transferring the load, resides more especially in thefact that an idle period which arises in known forms of constructionwith a following loadtransferring switch means cannot occur, because theswitching element which has selected the next tap contact and has thusprepared the change then changes its function and performs the transferof the load, so that no following movement of the other switchingelement takes place.

A structurally very simple solution is afiforded by embodying theinvention in such a way that for the selection of a tap the switchingelements of the movable contact assembly move parallel to and at adistance from the stationary tap contact bank and that for transferringthe load they are displaceable perpendicularly to the traversingmovement. According to the invention the independent traversingmovements and the load-transferring movements which are performedsubstantially perpendicularly to the traversing movement are eachproduced by separate drive means, the drive means for effecting the loadtransferring movement being a high-speed drive-to reduce wear ofcontacts by burning.

In order to achieve a satisfactory tap change within the minimum of timeit is preferred that the independently traversible switching elementsshould be mechanically or electrically coupled for the synchronousperformance of the load-transferring movement.

To reduce contact deterioration by burning it has also been foundadvisable to provide the high-speed drive for effecting theload-transferring movement with control means which determine theinstant of load transfer by reference to a phase shifting device in sucha manner that the transfer of the load will be effected approximatelywhen the current swings through zero.

As will be readily understood the on-load tap changer proposed by theinvention can be readily adapted to the control of multiphasetransformers in which the on-load tap change is effected in at least twotransformer phases. To this end the invention provides a pair ofswitching elements for each tapped transformer phase, which work insynchronism.

In an on-load tap changer for multiphase transformers with star pointvoltage control, in which the movable switching assembly forms the starpoint, the solution offered by the invention consists in providing eachof the switching elements with contact points equal in number to thenumber of phases joined at the star point, and in embodying theswitching elements in spider-like members which cooperate with astationary bank of tap contacts arranged concentrically about the commonaxis of the spiders.

A particularly compact design in a form of construction of this kind canbe achieved if the stationary concentric bank of tap contacts are formedwith contact faces on opposite sides and the two switching elements arearranged one on each side of said bank of tap contacts.

In an on-load tap changer in which the free ends of the independentlyrotatable switching elements, which cooperate with the bank contacts,are fitted with contact means comprising a main contact and a sparkingcontact, a further useful development of the invention consists inmaking the contact means independently movable perpendicularly to thedirection of rotation of the switching elements. Since in such anarrangement it becomes unnecessary to move the switching elements intheir entirety in order to break the connection with one tapping and tomake a connection with another tapping, the inertial forces which mustbe overcome for effecting the changeover can be substantially reducedand the speed of operation correspondingly increased.

The contact means on the two switching elements are preferably coupledand operated by a common drive means adapted to shift themperpendicularly to the bank contacts. In an onload tap changercomprising coaxial shafts for rotating the switching elements aparticularly satisfactory arrangement consists in utilizing the innershaft for the additional purpose of imparting the necessaryperpendicular movements to the two contact means by providing tiltablelevers on the switching elements for transmitting the axial movements ofthe inner shaft to the contact means.. p

These and numerous other features of the invention will now be describedin greater detail by reference to several illustrative embodiments shownin the accompanying drawings in which FIGS. 1 to 3 diagrammaticallyillustrate the manner of operation of an on-load tap changer for thecontrol of a single-phase transformer,

FIG. 4 is a longitudinal section of a practical embodiment of an on-loadtap changer according to FIGS. 1 to 3,

FIGS. 5 and 6 are sections taken on the lines V-V and VIVI in FIG. 4,

FIGS. 7 and 8 are detail sections (on a larger scale) of the switchingelements and tap contacts in a tap changer constructed as shown in FIGS.4 to 6;

FIG. 9 is a detail section of the switching elements and tap contacts inan alternative embodiment of the invention,

FIGS. 10 and 11 are schematic representations of an on-load tap changerfor a three-phase transformer,

FIGS. 12 and 13 are a section and plan view respectively of an on-loadstar point tap changer according to the invention for a three-phasetransformer,

FIGS. 14 to 16 and 17 to 19 are diagrammatic plan andelevational viewsof two three-phase transformers with built-in on-load tap changers ofdifferent forms of construction,

FIG. is a sectional representation of part of an onload tap changer in aform of construction representing a modification of that shown in FIG.4,

FIG. 21 is a section taken on the line XXVXXV in FIG. 20.

With reference now to FIGS. 1 and 2, these drawings diagrammaticallyillustrate the principle upon which the operation of the on-load tapchanger according to the invention is based. The tap changer 1 comprisestwo switching arms 2 and 3 which are independently rotatable aboutcoaxial vertical shafts 5 and 4, and turn in horizontal planes whichremain at the same vertical distance from one another. The free ends ofthe two switching arms 2 and 3 carry contacts 6 and 7 adapted tocooperate with contacts 8 connected with the transformer tappings.

These tap contacts 8 which are connected with the tappings 9 of thecontrolled transformer 10 (cf. FIG. 3)

are disposed in a circular bank about the two axes of rotation 4'and 5of switching arms 2 and 3.

One of the two switching arms 2 or 3 is now rotated into alignment aboveor below a selected tap contact 8. The actual change-over from theprevious tap to the freshly selected tap is then effected by lifting theswitching arm which still aligns with the preceding tap, thus isolatingthe latter, and at the same time pushing the other switching armvertically into contact with the freshly selected tap. To this end eachswitching arm 2 and 3 is associated with drive means 11 and 12 of itsown (cf. FIG. 3), and each arm can thus be independently rotated.

Moreover, the two switching arms 2 and 3 are equipped with means 13 forjointly displacing them in the vertical direction, in other words formoving both arms 2 and 3 together longitudinally in relation to theiraxes of rotation 4 and 5. These vertical displacing means permit the twoarms 2 and 3 to be moved into a position in which the contact point 6 or7 of one of the arms will make contact with a bank contact 8, whereasthe contact point of the other arm will be ata level at which no contactwith the bank of tap contacts is possible, but in which it can be swunginto alignment with the next bank contact that may be selected. Whenthis latter switching arm is thus rotated into register with the nexttap, a further joint axial displacement of both switching arms willcause contact with the next tap to be made and that with the precedingtap to be broken.

It follows from what has been said that the two switching armscontinually alternate in function inasmuch as one arm will make when theother arm breaks contact, whereas during the following change the lastmentioned arm will make and the first mentioned arm break, and so Aswill be clear from FIG. 3, and as will be later described in greaterdetail, the on-load tap changer 1 including its drive means 11, 12, 13and transmission 14 between drive means 12 and shaft, 4 can be combinedin an integral assembly with a transformer 10 which comprises windings15 and a core 16, by mounting the tap changer inside or on thetransformer casing.

FIGS. 4 to 6 show an embodiment of an on-load tap changing switchaccording to the invention for the control of a single-phasetransformer. This will now be described in greater detail.

The tap changer is housed in a casing 18 made of an insulating or otherappropriate material, and closed by a cover plate 19 bolted at 20 to aflange 21 secured to the casing 18 by angle irons 22 and rivets orscrews 23. A floor 24 with an oil drain cock 26 forms the bottom ofcasing 18 and is likewise secured by bolts or rivets 25.

Moreover, afiixed to cover 19 by bolts or rivets 27 is a hollow cylinder28 from which a pressure tight tank 29 is suspended by means of bolts orrivets 30.

The tap contacts 8 which carry contact faces St; on both sides projectinto this pressure-tight tank 29. As will be understood by referencemore particularly to FIG. 6 the tap contacts are disposed in a circularbank around the periphery of the cylindrical casing 18 and are connectedwith terminals 8b on the outside of the casing. Above and below thecircular bank of tap contacts 8 are the switching arms 2 and 3 rotatablyand vertically shiftably mounted inside the tank.

The upper switching arm 3 is secured to a hollow shaft 4 which can bedriven by a motor 12 through shaft 34, a transmission 32 and 33 and apin 31 projecting from shaft 4.

The lower switching arm 2 is secured by means of pin 35 to a shaft 5located inside the aforesaid hollow shaft .4 and adapted to be driven bya motor 11 through a transmission 36, 37.

To permit the switching arms 2 and 3 to be stepped from contact tocontact the two wheels 32 and 36 are formed as Geneva wheels, as shownclearly in FIG. 5. A pin 33b afiixed between two lobed extensions 3311on the cooperating wheel 33 engages slot-like recesses 32a in the wheel32 and, in the course of each full revolution of shaft 34, steps thewheel 32 by the pitch of two neighbouring slots. The two wheels 36 and37 in the transmission between motor 11 and shaft 5 are formed inexactly the same way as the wheels 32 and 33.

Vertical shift of shafts 4 and 5 (and hence of switching arms 2 and 3)is performed by a lifting mechanism 13 which, in the same way as the twomotors 11 and 12, is mounted on the cover plate 19 of the casing. Theaxial coupling between the two shafts 4 and 5, which are independentlyrotatable as has been explained, is effected, on the one hand (when theshafts are lowered), by a wider collar-like upper portion 5a of shaft 5which bears on a washer 38 on the upper end face of shaft 4 and, on theother hand (when the two shafts 4 and 5 are raised), by a lower Widerportion 5b of shaft 5 which bears on a washer 39 at the bottom end faceof hollow shaft 4.

When they are raised and lowered the two shafts are guided in that thebottom end 5b of shaft 5 slidably moves in a bush 40 aflixed by aspider-shaped bracket 41 by means of screws 42 to the pressure-tighttank 29.

This spider 41 together with the hollow cylindrical ring 43 at the upperend of casing 29 simultaneously serves to reinforce and stiifen thewalls of the tank. The top of tank 29 is closed by a plate 44 whichcarries bearings for shaft 34 and a bearing plate 46 for mounting thestar wheel 32.

As will be seen by reference to FIG. 6 the switching arms 2 and 3 arerelatively narrow bars fitted with contact elements for cooperation withthe bank of tap contacts 8. These contact elements consist of a movablesparking contact 47 and a main contact 48 formed as a flat annularplate. The sparking contact 47 which is insulatedly slidable in relationto the main contact 48 is guided by its shaft 47a of which the free endis connected with a resistor 49. Each resistor 49 which forms a divertoris connected by a clip 50 with a metal plate 51 which is mounted onswitching arms 2 and 3 respectively and with which the main contact 48makes a good metal to metal contact.

The main contacts 48 of the two switching arms 2 and 3 areinterconnected by a solid metal loop 52a and a flexible conductor 52bleads to a terminal 53 which projects through casing 18 and casing 29.

The manner in which this on-load tap changer functions will at once beclear by reference to the drawings. In the position of the switchingelements shown in FIG. 4 shafts 4 and 5 are in their raised position, inwhich the lower switching arm 2 makes contact with one of the bankcontacts 8. The sparking contact 47 of the lower arm 2 is fullydepressed so that the circuit from terminal 53 is closed through cable52b, plate 51, main contact 48 and sparking contact 47 of arm 2 to theselected bank contact 8. Assuming the tap is to be changed, then arm 3will first be rotated into register with the neighbouring contact 8. Thevertical shifting means 13 (for instance in the form of a double-actingsolenoid and plunger) then lowers the entire switching assemblycomprising shafts 4 and 5 and the two arms 2 and 3. This motion firstcauses the sparking contact 47 on the lower arm 2 to separate from themain contact 48, resulting in the introduction of resistor 49 into thecircuit. In the course of the further downward movement of the selectorassembly the sparking contact 47 on the upper arm 3 will make contactwith the neighbouring bank contact 8. Consequently a fresh circuit isnow completed from terminal 53 through cable 52b, loop 52a, resistor 49and sparking contact 47 on the upper arm 3 to the neighbouring tap. Fora brief instant the transformer winding between the two taps willtherefore be short-circuited through the two resistors 49 of switchingarms 2 and 3. In the course of the continued downward motion of shafts 4and 5 the sparking contact 47 on the lower arm 2 will separate .from thebank contact 8 which had so far been in circuit and current will flowexclusively through the fresh bank contact 8. In the final stage of thedescent of shafts 4 and 5 sparking contact 47 on the upper arm 3 will bedepressed until the current can flow through the main contact 48 of arm3, the divertor resistor 49 being again short-circuited.

If the tapping position thus attained is not yet the desired position,then the tap change will be repeated in the manner that has beendescribed. The two arms 2 and 3 function turn and turn about, each armin turn preparing the change and taking over the load from the otherarm.

.In the upper portion of casing 18 contacts 55 are provided (of. FIGS. 4and 5) for optional use. These are electrically connected through an arm56 with the outer hollow shaft 4.

Details of the manner in which the contacts are mounted on switchingarms 2 and 3 as well as the disposition and manner of attachment of thebank contacts are illustrated in FIGS. 7 and 8. The bank contacts 8which are provided with annular contact elements 8a are secured byflange-like fixing members 57 with rounded edges; the

8 members 57 are secured by bolts 58 to the wall of chamber 29.

The sparking contacts 47 have shafts 47a which are slidably held in theswitching arms 2 and 3 in insulated manner. Insulation is provided byinsulating bushings 59, 60 and 61 each provided with a sliding passagefor the shaft 47a of the sparking contact 47. Inserted into the middleinsulating bushing 60 is a metal plate 62 which intercepts a collar 47bon the shaft of the sparking contact 47 The annular main contacts 48 arefitted into the metal plate members 51 of arms 2 and 3. Each maincontact 48 encircles shaft 47a of the associated sparking contact. Inthe working position illustrated in FIG. 7 the current flows from plate51 through main contact 48, sparking contact 47, to the bank contact 8.If arm 2 is lowered, sparking contact 47 will be lifted off the maincontact by the action of a compression spring 63. When this happens theabove mentioned divertor resistor 49 (of. FIG. 4) will be cut into thecircuit. However, in addition to compression spring 63, another springacts on the intermediate plate 64 which carries the insulating bushing60, said additional spring 65 enveloping a threaded pin 66. The sparkingcontact 47 will therefore rise when switch arm 2 breaks the circuituntil the compressional forces of springs 63 and 65 are in equilibrium.

An alternative arrangement of the contact elements on switching arms 2and 3 is shown in FIG. 9. Whereas in the example shown in FIG. 7 thesparking contacts 47 are embodied in plates which in operative positionbear down on the ring-shaped main contact 48 so that the working currentmust flow through the sparking contact plates, the sparking contacts 47in FIG. 9 are domeheaded contacts which are completely displaced intothe interior of the ring-shaped main contacts 48 when arms 2 and 3 arein make position. This form of construction has the advantage that theworking current passes directly from the bank contact 8 into maincontact 48 on the relative switching arm, whereas the sparking contacts,which after prolonged periods of service always show signs of wear, donot carry the full load but merely work in parallel with the maincontact 48. Since the divertor resistor 49 lies in series with thesparking contact practically the full load will be carried by the maincontacts 48.

FIGS. 10 and 11 schematically illustrate two alternative arrangementsfor a tap changer according to the invention for use in conjunction witha three-phase transformer. In the tap changer shown in FIG. 10 theswitching arms 2a, 2b, 2c and 3a, 3b, 3c respectively which areassociated with the three phases are arranged in tiers the one above theother and they can be jointly raised and lowered by the two shafts 4 and5. The tap contacts 8 associated with the several phases are in thiscase correspondingly disposed in three tiered banks.

In the alternative example shown in FIG. 11 a separate on-load tapchanger (each with a pair of movable switching arms 2 and 3) isassociated with each of the three phases. The switching arms 2 and 3 arecollectively operated by common actuating means 67 and 68 connected bychain transmissions or other drive means with the switching arms 2 and 3of each tap changer assembly. In its fundamental construction and in themanner of its operation this three-phase on-load tapchanger iscompletely analogous to the single phase tap changers that have alreadybeen described. If it is desired that the onload tap changers associatedwith each of the three phases should effect the change when the currentpasses through zero, then separate drive means must naturally beassociated with each of the tap changing assemblies 111, 1b, 1c and twoindependently operable motors 67, 68 must be provided for each changer.

Whereas in the illustrative embodiment shown in FIG. 10 the tap contactsassociated with the three phases of the transformer were arranged intiers and in FIG. 11 in 7 separate assemblies side by side, the tapcontacts 8 in a form of construction that will now be described arearranged in a'single circular bank, a sector of 120 being associatedwith each phase.

In the on-load tap changer shown in FIGS. 12 and 13 the switching arms 2and 3 are arranged to form the star point. Consequently they are formedwith three electrically connected arms 2d, 2e, 2 and 3d, 3e, 3respectively, each arm cooperating with one of the three sectors 70, 71,72. The bank contacts 8 in each of the sectors 70, 71, 72 areconnectedwith the star point tappings in the three phases of the transformer.

Thearms of switching members 2 and 3 which-see FIG. 12are slightlyarched towards the bank of tap contacts 8 carry insulated and freelyslidably movable sparking contacts 47 at their ends as well as maincontacts 48 which are electrically connected with the arms themselves.Divertor resistances 49 interposed between the sparking contacts 47 andthe switching arms 2 and 3 are affixed to the underside of each arm 20!,26, 2f and 3d, 3e, 3f respectively.

The switching arms 2 and 3 are joined to a common hub 73 and 74 which isfast ona shaft 75 and 76, respectively. These shafts 75 and 76 can bestepped from tap to tap as required by driving means similar to thosedescribed previously. The vertical shift of the switching arms requiredfor each change from one tap to the next is effected by twoliftingsolenoids 77 and 78 which are arranged at opposite ends of each shaftand act in contrary directions. To ensure that the switching arms 2 areraised from the bank of tap contacts 8 at high speed a tension spring 79attached to one end of each shaft 75, 76 is stretched when theassociated arm makes contact with the bank contacts, md then powerfullyassists the opening action of the lifting solenoids 78. In order torelieve the solenoids 77 when the switching arms are in the closingposition, arresting means 80 are provided which automatically retain thearms 2 and 3 in the closing (i.e. contacting) position, but release theshafts 75 and 76 as soon as a lifting solenoid 78 is energized.

In the embodiment illustrated in FIGS. 12 and 13 the sparking contacts47 are slidably held in insulating bushes 81 and are retained indiverting the position, when the switching arms are in the raised(non-contacting) position, by leaf springs 82 which bear against theends of the sparking contact shafts 47a.

The necessary synchronization of the vertical movement of switching arms2 and 3 (which in the previous embodiments was assured by the rigidaxial coupling of shafts 4 and is achieved in the present example, shownin FIGS. 12 and 13, by operating the lifting solenoids 77 and 78 of botharms from the same controlling source of current S and S respectively.

This embodiment functions in a manner that is entirely analogous to theforms of construction already described. Again the two switchingelements 2 and 3 operate alternately to prepare the change and take overthe load, the individual change being effected through the sparkingcontacts 47 and the divertor resistors 49 in such a way that the circuitis never interrupted and individual winding. sections of the transformerare never directly shortcircuited.

The following FIGS. 14 to 19 finally illustrate two ways of disposing anon-load tap changer according to the invention in a transformer tank,the illustrations being very much simplified and somewhat diagrammatic.

The three-phase transformer shown in FIGS. 14 to 18 in tank 83 comprisesa three-limbed core 84 which carries the three phase windings 85, 86 and87. Adjacent the core 84 in a lateral extension 83a of the transformertank are the on-load tap changers 1a, 1b, which are associated with thethree transformer phases. The tap changer units 1a, 1b, 1c are arrangedat the corners of a triangle and each is provided with drive means 88a,88b, 880 of it own which project from the bottom of the tank 83 andwhich are therefore easily accessible when the transformer is inoperation.

In the embodiment according to FIGS. 17 to 19 the tap changer unitsassociated with the three transformer phases are arranged the one abovethe other in the manner schematically illustrated in FIG. 10. The tapchanger thus has the overall shape of an elongated cylinder which islocated adjacent the core 34 of the transformer. The longitudinaldimensions of the transformer tank 83 will therefore be somewhat shorterthan in the previous example. Again the drive means 88 of the tapchanger 1 project from the bottom of the transformer tank 83.

Finally, FIGS. 20 and 21 illustrate yet another embodiment of theinvention which represents a particularly advantageous practical form ofconstruction both from the production and operational points of view.

The switching arms 2 and 3in a manner analogous to the previouslydescribed embodiment of the invention are fast on the two independentlyrotatable coaxial shafts 5 and 4. The free ends of the switching arms 2and 3 carry contacting devices which substantially comprise aninsulated, vertically displaceable sparking contact 47 and a maincontact 48. The contact assernblies 100 are themselves freely verticallydisplaceable in cylindrical guide ways 101 at the outer ends ofswitching arms 2 and 3. In the embodiment shown in FIGS. 20 and2l-contrary to the previously described forms of construction-theswitching arms 2 and 3 are not themselves contrived to be movedperpendicularly to their plane of rotation. This movement is performedonly by the contact assemblies 106 which naturally represent a muchlighter weight and are thus capable of being moved with a much smallereffort.

To this end two levers 102 are provided whichcf. FIG. 21have relativelyoffset ends. The centre portion of these levers has bosses 193 whichfulcrum on a pin 104 between the sides of each switching arm 2 and 3.The outer end of said levers engages the contact assemblies 100 by meansof a pin 1% and slot 105 motion. The inner end of each lever 102, ofwhich both ends are bifurcated, is provided with inward projections 107which engage an annular slideway on the inner shaft 5, said slidewaybeing formed by two closely adjacent flangelike collars 1118. The levers102 are thus vertically tiltably connected with shaft 5 Without at thesame time interfering with the rotatability of the shaft.

The on-load tap change in this embodiment actually proceeds in exactlythe same way as has been explained by reference to FIG. 4. The twoswitching arms 2 and 3 alternately perform the function of selecting afresh contact 8 and taking over the load, and the verticaldisplaceability of the contact assemblies whilst the switch arms remainrigid ensures that tap changing can be performed at very high speed.

In this form of construction provision is likewise made to permit thetank containing the switching assembly and the bank contacts 8 to befreely lifted out of the casing 18 which is fitted with the tappingterminals 53.

What I claim is:

l. A tap change mechanism which comprises stationary tap contactsarranged in a plane and uniformly distributed round the circumference ofa circle, two spiders each having the same number of arms and eacharranged on a respective side of said plane and mounted on a respectiverotatable shaft extending at right angles to said plane, and switchingelements supported on the ends of the arms, and in which said shafts arerotatable independently of one another and axially movablesimultaneously with one another and in the same direction to bring theswitching elements on each of said spiders respectively into and out ofcontact with said tap contacts, and in which said spiders are arched todefine spaces between said spiders and said plane, and which furthercomprises separate electromagnetic means, accommodated in said spaces,for moving each of said shafts towards said plane, arresting means, alsoaccommodated in said spaces, for retaining each of said spiders inpositions in which said switching elements on said spiders are incontact with said tap contacts, and further separate electromagneticmeans for moving each of said shafts away from said plane.

2. A tap change mechanism which comprises stationary tap contactsarranged in a plane and uniformly distributed round the circumference ofa circle, a stationary terminal, a rotatable hollow shaft extendingthrough and at right angles to said plane, two arms located one on eachside of said plane and rigidly secured to said hollow shaft, twoswitching elements each carried by a respective one of said arms andslidable in a direction at right angles to said plane, an axiallymovable inner shaft extending parallel to and inside said hollow shaft,and two levers each fulcrumed on a respective one of said arms andhaving an inner end pivotally and slidably connected to said inner shaftand an outer end connected by a pin and slot motion to the switchingelement on said one of said arms, and which further comprises arespective resistor for each of said switching elements, and in whicheach switching element comprises a sparking contact permanentlyconnected to the stationary terminal by conductor means independent ofthe resistor, said sparking contact being insulated from said maincontact and slidable relatively to said main contact and said tapcontacts, and resilient means urging said sparking contact into aposition between said tap contacts and said main contact.

3. A tap change mechanism as claimed in claim 2, in which the centralshaft has two pairs of adjacent collars defining circular guideways, andeach end of each lever is bifurcated, the inner end of each lever havingprojections engaging in one of said guideways.

4. A tap change mechanism which comprises a stationary row of tapcontacts arranged in a plane, a stationary terminal, a pair of switchingelements permanently electrically connected to the stationary terminaland disposed one on each side of said plane and movable independently ofone another parallel to said plane and movable simultaneously with oneanother in each of two opposite directions at right angles to saidplane, whereby simultaneous movement of both of said switching elementsin either one of said directions can bring one of said elements intocontact with one of said tap contacts in said row and the other one ofsaid elements out of contact with one of said tap contacts in said row,and which further comprises a respective resistor for each of saidswitching elements, and in which each switching element comprises asparking contact permanently electrically connected to the terminalthrough the resistor, a main contact permanently connected to thestationary terminal by conductor means independent of the resistor, saidsparking contact being insulated from said main contact and slidablerelatively to said main contact and said top contacts, and resilientmeans urging said sparking contact into a position between said row oftap contacts and said main contact.

References Cited in the file of this patent UNITED STATES PATENTS1,366,731 Hoescher Jan. 25, 1921 1,678,129 Tuttle July 24, 19281,709,724 Hill Apr. 16, 1929 1,867,147 Haller July 12, 1932 1,931,869Jansen Oct. 24, 1933 1,985,927 Jansen Jan. 1, 1935 2,009,383 Blume July30, 1935 2,026,356 Palme Dec. 31, 1935 2,046,979 Turner July 7, 19362,222,719 Prince Nov. 26, 1940 2,231,627 Jansen Feb. 11, 1941 2,435,438Fowler Feb. 3, 1948 2,470,625 Kuhn May 17, 1949 2,513,677 Rigert July 4,1950 2,675,519 Aicher Apr. 13, 1954 2,680,164 Lennox June 1, 19542,680,790 Jansen June 8, 1954 2,915,720 Mueller et a1. Dec. 1, 19593,014,999 Pensis Dec. 26, 1961 FOREIGN PATENTS 290,450 Germany Feb. 28,1916 915,242 Germany Nov. 2, 1954 338,081 Great Britain Nov. 13, 1930764,405 France May 1, 1934 1,051,190 France Sept. 9, 1953 1,087,752France Sept. 1, 1954

1. A TAP CHANGE MECHANISM WHICH COMPRISES STATIONARY TAP CONTACTSARRANGED IN A PLANE AND UNIFORMLY DISTRIBUTED ROUND THE CIRCUMFERENCE OFA CIRCLE, TWO SPIDERS EACH HAVING THE SAME NUMBER OF ARMS AND EACHARRANGED ON A RESPECTIVE SIDE OF SAID PLANE AND MOUNTED ON A RESPECTIVEROTATABLY SHAFT EXTENDING AT RIGHT ANGLES TO SAID PLANE, AND SWITCHINGELEMENTS SUPPORTED ON THE ENDS OF THE ARMS, AND IN WHICH SAID SHAFTS AREROTATABLE INDEPENDENTLY OF ONE ANOTHER AND AXIALLY MOVABLESIMULTANEOUSLY WITH ONE ANOTHER AND IN THE SAME DIRECTION TO BRING THESWITCHING ELEMENTS ON EACH OF SAID SPIDERS RESPECTIVELY INTO AND OUT OFCONTACT WITH SAID TAP CONTACTS, AND IN WHICH SAID SPIDERS ARE ARCHED TODEFINE SPACES BETWEEN SAID SPIDERS AND SAID PLANE, AND WHICH FURTHERCOMPRISES SEPARATE ELECTROMAGNETIC MEANS, ACCOMMODATED IN SAID SPACES,FOR MOVING EACH OF SAID SHAFTS TOWARDS SAID PLANE, ARRESTING MEANS, ALSOACCOMMODATED IN SAID SPACES, FOR RETAINING EACH OF SAID SPIDERS INPOSITIONS IN WHICH SAID SWITCHING ELEMENTS ON SAID SPIDERS ARE INCONTACT WITH SAID TAP CONTACTS, AND FURTHER SEPARATE ELECTROMAGNETICMEANS FOR MOVING EACH OF SAID SHAFTS AWAY FROM SAID PLANE.